Materials and methods for biosynthesis of serine and serine-related products

ABSTRACT

Engineered DNA encoding 3-phosphoglycerate dehydrogenate (PGD) has reduced sensitivity to inhibition by serine in comparison to wild-type PGD. The DNA encodes PGD which has at least some level of enzymatic activity useful for biosynthesis, and which retains that activity at higher serine levels than does the (unmodified) wild-type PGD. The PGD has the amino acid sequence of this above-described engineered DNA. Expression vectors contain this engineered DNA and regulatory DNA positioned and oriented for expression of the engineered DNA in a host expression system. Cells contain these expression vectors, and a method for producing serine or a serine-derived product includes culturing such cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the general field of biosynthesis of serine and products related to serine, particularly tryptophan, and to methods and materials used in that biosynthesis.

2. The Prior Art

Serine is a primary intermediate in the biosynthesis of a wide variety of cellular metabolites including such economically important compounds as choline, glycine, cysteine and tryptophan. In addition, serine acts as a single carbon donor and is responsible for 60% to 75% of the total need of the cell for C₁ units through the production of 5,10-methylenetetrahydrofolate from tetrahydrofolate. These C₁ units are used in a wide variety of biosynthetic pathways including the synthesis of methionine, inosine monophosphate, other purines and some pyrimidines (e.g., thymidine and hydroxymethyl cytidine).

The serine biosynthetic pathway shown in FIG. 1 is generally available to a wide variety of tissues and microorganisms. The first committed step in that pathway is the conversion of 3-phospho-D-glyceric acid (PGA) to 3-phosphohydroxypyruvic acid (PHA) by means of the enzyme 3-phosphoglycerate dehydrogenase (PGD). The gene encoding PGD has been cloned and sequenced, and the amino acid sequence of the PGD subunit has been deduced. Tobey and Grant, J. Biol. Chem., 261:12179-12183 (1980).

In procaryotes (particularly bacteria) and microorganisms such as yeast, but not in higher eukaryotes, activity of wild-type PG is inhibited by cellular serine levels. This inhibition has been studied kinetically and reportedly proceeds in an allosteric manner. Tobey and Grant, J. Biol. Chem., 261:12179-12183 (1986); Dubrow and Pizer, J. Biol. Chem., 252:1527-1551 (1977); McKitrick and Pizer, J. Bacteriol., 141:235-245 (1980).

Tosa and Pizer, J. Bacteriol., 106:972-982 (1971), studied the effect of a normally toxic serine analog, L-serine hydroxamate, on an E. coli strain. Selection on a growth medium containing that analog yielded serine-resistant mutants. Some mutants were shown to have a modification in an enzyme unrelated to PGD, seryl-tRNA synthetase. Crude extract of one mutant showed PGD activity with reduced serine sensitivity (See, J. Bacteriol., 106:972-982 [1971]; FIG. 5; Table 6; and see p. 973 bottom left col., p. 977 bottom left col.).

SUMMARY OF THE INVENTION

One aspect of the invention generally features DNA encoding 3-phosphoglycerate dehydrogenase (PGD) with reduced sensitivity to inhibition by serine in comparison to wild-type PGD, i.e., the DNA encodes PGD which has at least some level of enzymatic activity useful for biosynthesis, and which retains that activity at higher serine levels than does the (unmodified) wild-type PGD.

In preferred embodiments, the wild-type PGD is microbial or yeast PGD. Also preferably, the engineered DNA encodes PGD which comprises an alteration in the C-terminal 25% of wild-type PGD, preferably in the C-terminal 50 amino acids. For example, the engineered DNA may encode PGD comprising a deletion in part of all of the C-terminus. Also preferably, the engineered DNA encodes PGD having an insertion in the C-terminus (e.g., between VAL 363 and ASN 364, or between ALA 392 and GLN 394) in addition to the deletion described above, or as a separate alteration.

The present invention is also directed to: a) PGD having the amino acid sequence of the above-described engineered DNA; b) expression vectors comprising the engineered DNA and regulatory DNA positioned and oriented for expression of the engineered DNA in a host expression system; c) cells comprising such expression vectors; and d) methods for producing serine or a serine-derived product by culturing such cells. As to c) above, the cell preferably is deleted for wild-type serA.

Yet another embodiment generally features a cell engineered (e.g., it includes a recombinant genetic construction) to produce a PGD-encoding mRNA transcript with an altered 3' end, which transcript is translated by the cell to yield PGD with reduced sensitivity to inhibition by serine in comparison to wild-type PGD.

The present invention provides decontrol of an important biosynthetic control point, thereby enhancing production of numerous compounds downstream of that point, including, in particular, serine and serine-derived products such as tryptophan. Other cellular metabolites derived from serine (i.e., serine is a primary intermediate in their biosynthesis) include choline, glycine, cysteine and C₁ -donor-dependent compounds such as methionine, inosine monophosphate, purines, and some pyrimidines (e.g., thymidine and hydroxymethyl cytosine).

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings, which disclose the embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.

In the drawings, wherein similar reference characters denote similar elements throughout the several views:

FIG. 1 shows the steps in the biosynthesis of L-serine from glucose;

FIGS. 2(1)-2(5) show the sequence of the E. coli serA gene reported by Tobey and Grant (cited above) and the amino acid sequence deduced from the gene; and

FIG. 3 depicts bioconversion of L-serine and tetrahydrofolate to glycine and N⁵,N¹⁰ -methylene tetrahydrofolate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Providing Serine-Insensitive PGD

1. Genetically Engineered Constructions.

The preferred embodiments of the invention feature biosynthesis of serine and serine-related products, e.g., products described above derived by biosynthesis from serine. A first step in biosynthesis of these compounds according to the invention is the provision of serine-insensitive PGD, as discussed below.

It has been determined that there is a specific serine feedback mediating domain in PGD, and that domain can be altered to reduce serine sensitivity while maintaining useful levels of PGD activity. FIG. 2 shows one particular PGD genetic and amino acid sequence (SEQ ID NO: 1) which can be used for reference in the following discussion. The sequence of FIG. 2 (SEQ ID NO: 1) includes 410 amino acids (including the initial Met which is cleaved from the mature protein). The domain of PGD that can reduce serine sensitivity, without destroying PGD activity, is within the C-terminal 25% of the molecule, most preferably the 50 C-terminal residues.

Examples of PGD modifications that fall within the invention are deletions of some or all of the C-terminal 42 amino acids, or insertions or substitutions within that region which reduce serine insensitivity while retaining useful PGD function. For example, insertion of amino acid residues between Val 363 and Asn 364 will increase the K_(i) of the PGD over wild-type, while retaining PGD activity.

More dramatic increases in K_(i) are accomplished by deleting some or all of the C-terminal amino acid residues. For example, deletion of the C-terminal residues GTIRARLLY and replacement with ASLD increases K_(i) by several orders of magnitude, while retaining a useful level of PGD activity. Other insertions within the scope of the invention are insertions between Ala 392 and Gln 394.

Other useful modifications include deletions from the C-terminus in addition to the insertions and modifications discussed above.

Genes encoding serine-insensitive PGD described above can be constructed by genetic engineering techniques that involve altering the 3' end of the coding region coding for the C-terminal amino acids, and then transforming a host strain with a vehicle to express the altered PGD enzyme. Candidate altered enzymes are screened (as described below) for serine affinity (K_(i)) and for PGD activity by the methods generally discussed below.

2. Screening the Genetically Engineered Constructions.

In screening genetic constructions made by the above-described methods, the following assays of PGD activity and of serine sensitivity are used.

While not critical to the invention, the assay of PGD activity is generally necessary in order to establish the degree of serine sensitivity of the altered enzyme. As is well known in the art, enzyme activity is a function of the total number of enzyme molecules and the catalytic activity of each molecule. Thus, in comparing the catalytic activity of PGD feedback variants, steps must be taken to adequately control for the relative number of PGD molecules for samples in which relative catalytic activity is to be compared. There are a number of ways in which this may be accomplished. However, since it is difficult to adequately establish the level of gene expression in cells transformed with truncated serA genes (due to decreased viability), the most suitable way to compare PGD activity produced from various constructs and the wild type is to chromosomally integrate the altered serA gene containing standard regulatory elements in a single copy, followed by harvesting the transformants and determination of the relative catalytic activity as compared to PGD from wild type cells.

Any method suitable for the measurement of PGD activity may be employed. PGD activity may be measured through detection of either the forward or the reverse reaction by the method of John C. McKitrick and Lewis I. Pizer, J. Bacteriol., 141:235-245 (1980).

The enzymatic assay described above is suitable for determination of serine sensitivity for any PGD enzyme, including those with chemically modified C-termini. The assay is performed in the presence of various levels of serine. The catalytic activity in the presence of serine is compared to catalytic activity in the absence of serine, and the K_(i) calculated.

In most cases it will be preferred to reduce serine sensitivity without significantly altering PGD catalytic activity. In still other embodiments it may be desirable to reduce both the feedback sensitivity and the catalytic activity. The constructions having a C-terminal amino acid sequence of 3-phosphoglycerate dehydrogenases listed in TABLE 1 (described below) may be used.

                  TABLE 1                                                          ______________________________________                                         C-terminal Amino Acid Sequences of                                             3-phosphoglycerate dehydrogenases                                              serA      Sequence       ki/mM   Units                                         ______________________________________                                         WT        (SEQ ID NO: 2) <0.1    .05                                           1455      (SEQ ID NO: 3) >100    <.01                                          1459      (SEQ ID NO: 4) >100    N/A                                           1507      (SEQ ID NO: 5) >100    N/A                                           1508      (SEQ ID NO: 6) 3.8     0.5                                           1509      (SEQ ID NO: 7) >100    N/A                                           1510      (SEQ ID NO: 8) >100    N/A                                           1511      (SEQ ID NO: 9) >100    N/A                                           1512      (SEQ ID NO: 10)                                                                               >100    N/A                                           1530      (SEQ ID NO: 11)                                                                               >100    N/A                                           1531      (SEQ ID NO: 12)                                                                               >100    N/A                                           ______________________________________                                    

Other constructions with modified 3' ends also fall within the scope of the present invention since it is a simple matter to prepare test constructs and transform cells according to the present invention and test for serine inhibition of PGD activity.

Any vector which leads to expression of a PGD protein lacking sensitivity to inhibition by serine pertains to the present invention. In general, however, in the absence of a sink for serine, high levels of expression of feedback free PGD should be avoided since the resulting high cytoplasmic levels of serine or serine-derived metabolites can be toxic to the cell. Thus, in general, for any construct coding for a feedback inhibited PGD with normal catalytic activity and expression levels similar to those from the native gene, transformation will likely lead to high levels of PGD expression and decreased cellular viability. The toxicity of high levels of serine produced may, in fact, select for mutants with decreased PGD expression. Thus, while transformation using multi-copy plasmids may be useful in initial screening of constructs with some embodiments, it is preferred to chromosomally integrate serA constructs in single copies into the genome. Additionally, chromosomal integration as described below facilitates activity measurement of the feedback deleted PGD. Thus, in most embodiments where strong catalytic activity is expected or desired, it is preferred to utilize vectors suitable for single copy chromosomal integration. Many such vectors and strategies for their use are known in the art. See, e.g., Backman, U.S. Ser. No. 07/091,837, filed Sep. 1, 1987, hereby incorporated by reference. Useful vectors and constructs can be made to allow for the successful transformation and expression of the enzyme in an appropriate host for producing the desired product. Means for accomplishing these ends are well known to those familiar with the art and are not central to the present invention. In addition to the altered PGD-encoding DNA, the expression vector will contain various other elements described below.

First, the coding sequences present on the vector will be accompanied by the appropriate regulatory elements necessary for the appropriate level of expression of the coding sequences, including promoters, ribosome binding sites, and termination sequences. In most cases, the native serA regulatory sequences will be the preferred source of the catalytically active part of the molecule, although it is recognized that many other regulatory sequences known to the art or yet to be discovered may be employed.

Second, it is preferred that sequences encoding selective markers and/or reporter genes, along with the appropriate regulatory elements, will also be present on the vector. The expression of such selective markers is useful in identifying transformants. Appropriate selective marker genes include those coding for ampicillin, tetracycline, and chloramphenicol.

Third, the desirability of an origin of replication on the plasmid vector depends largely on the desirability of maintaining the genes chromosomally or extrachromosomally. Those familiar with the art appreciate the various strategies by which the lack of an origin of replication can be exploited to promote integration into the chromosome. See, e.g., Backman et al, U.S. Pat. No. 4,743,546, hereby incorporated by reference.

Once the expression vector is constructed, a suitable host cell can be transformed with a vector containing a transcription unit coding for a serine insensitive PGD protein. In most cases, it is useful to employ cells for which the endogenous PGD protein is known to be inhibited by serine and in which the endogenous serA gene is deleted and replaced by the altered gene of the invention. Such cell systems are useful for the overproduction of serine-related metabolites. Cells known to contain serine sensitive proteins are prokaryotes and yeasts.

The following example illustrates, but does not limit, the invention.

EXAMPLE 1

Construction of serA Gene Alleles Encoding Feedback Resistant 3-phosphoglycerate Dehydrogenases

The E. Coli K12 serA gene was isolated on a 6.4 Kb DNA fragment from a Sau3A partial digest cloned into the BclI site of pTR264. See, Backman et al., U.S. Ser. No. 07/285,128, filed Dec. 16, 1988; and Roberts et al., Gene, 12:123 (1980). This plasmid was named pKB1302. A 3 Kb SalI to SphI fragment of pKB1302 DNA containing the serA gene was cloned into pUC19 to generated pKB1321. pKB1370 was generated by cloning a 3 Kb HindIII to SalI fragment containing the serA gene into pBR322.

Alleles of serA encoding feedback resistant 3-phosphoglycerate dehydrogenases were generated by inserting XbaI linkers at restriction sites in the 3' region of the serA gene. A partial digest of plasmid pKB1321 by HincII yielded blunt ends at position 1793, where insertion of linkers gave:

a) pKB1459, encoding a truncated 3-phosphoglycerate dehydrogenase; b) pKB1507, encoding a truncated 3-phosphoglycerate dehydrogenase; and c) pKB1508 which encodes a 3-phosphoglycerate dehydrogenase with a four amino acid residue insert.

PstI digestion of pKB 1321 gives 3' overhang at position 1888. Blunt ends were generated by the action of the Klenow fragment of DNA polymerase I. Linkers were ligated into the blunt end fragments and the derived plasmids were pKB1509 which encodes a 3-phosphoglycerate dehydrogenase with a two-amino-acid insert and pKB1510 which encodes a truncated 3-phosphoglycerate dehydrogenase. A KpnI digest of pKB1370 was made blunt ended with Klenow fragment of DNA polymerase I and inserted linkers yielded plasmids encoding truncated 3-phosphoglycerate dehydrogenase, pKB1455 and pKB1512, or 3-phosphoglycerate dehydrogenase with a two amino acid residue insert, pKB1511. Deletion plasmids pKB1530 and pKB1531 were generated by inserting the 0.8 Kb BamHI to XbaI fragment from pKB1508 or the 0.9 Kb BamHI to XbaI fragment from pKB1509, respectively, into the 5.8 Kb BamHI to XbaI fragment of pKB1511.

The following TABLE 2 summarizes the various constructs made.

                                      TABLE 2                                      __________________________________________________________________________     serA              Restriction                                                  Allele                                                                               Plasmid     Site     Linker   Result                                     __________________________________________________________________________     Ser A 1455                                                                           pKB 1370    Kpn I    SEQ ID NO: 13                                                                           Truncated                                  Ser A 1459                                                                           pKB 1321    Hind II  SEQ ID NO: 13                                                                           Truncated                                  Ser A 1507                                                                           pKB 1321    Hind II  SEQ ID NO: 14                                                                           Truncated                                  Ser A 1508                                                                           pKB 1321    Hind II  SEQ ID NO: 15                                                                           Insert                                     Ser A 1509                                                                           pKB 1321    Pst I    SEQ ID NO: 16                                                                           Insert                                     Ser A 1510                                                                           pKB 1321    Pst I    SEQ ID NO: 15                                                                           Truncated                                  Ser A 1511                                                                           pKB 1370    Kpn I    SEQ ID NO: 17                                                                           Insert                                     Ser A 1512                                                                           pKB 1370    Kpn I    SEQ ID NO: 18                                                                           Truncated                                  Ser A 1530                                                                           pKB 1511 + pKB 1508                                                                        Hind II + Kpn I   Deleted                                    Ser A 1531                                                                           pKB 1511 + pKB 1509                                                                        Pst I + Kpn I     Deleted                                    __________________________________________________________________________

For all the constructs, the starting vector, the restriction site used, and the sequence of the inserted linker are indicated. The Ki values for serine are given in Table 1, as well as the relative catalytic activity for three of these constructs following chromosomal integration (described below). N/A indicates that the construction was not chromosomally integrated, and the activity level therefore was not standardized.

3. Chemical Modifications

Those skilled in the art will understand that deletions or modifications of the C-terminus of wild-type PGD can be accomplished enzymatically or chemically, e.g., by various carboxypeptidases, including carboxypeptidase Y or by lactoperoxidase mediated iodination.

4. Use of Antisense mRNA

Alternatively, it may be possible to reduce serine sensitivity in vivo through the generation of PGD-encoding transcripts truncated at the 3' end by means of the producing antisense mRNAs that include nucleotide sequences complementary to portions of the 3' coding region of native or transformed PGD coding sequences.

C. Production of Desired Compounds

As shown in FIG. 3, serine is an intermediate in the production of glycine. It is also an intermediate in the production of N⁵ N¹⁰ -methylenetetrahydrofolate which is generalized C₁ donor essential for synthesis of methionine, purines (including inosine) and some pyrimidines. Thus, the over-production of serine from phosphoglycerate may be useful in a wide range of bacterial production systems, including production systems for choline, glycine, cysteine, methionine, tryptophan, and al purines, including inosine monophosphate.

The following specific examples further illustrate the invention.

EXAMPLE 2

Host Strain Preparation

Sequences internal to a plasmid born serA gene were replaced with a kanamycin resistance gene. This plasmid was then used to inactivate the host strain serA gene by means of allele exchange, as follows.

The serA region of YMC9 (ATCC33920) was cloned from chromosomal DNA, partially digested with Sau 3AI, by complementation of PC1523 (argI61, argF58, serA27, purA54, thr-25, tonA49, relA1, spoT1), obtained from Coli Genetic Stock Center, Yale University, New Haven, Conn. A 3 kb fragment carrying the serA gene was subcloned into pUC19, giving rise to a plasmid called pKB1321. From this plasmid, a 3 kb SalI to HindIII fragment was recloned into pBR322, giving rise to plasmid pKB1370. The KpnI site at the 3' end of the serA gene was converted to BamHI with a linker and the BamHI fragment internal to the resulting serA was replaced with the BamHI fragment from pUC-4-KSAC (Pharmacia) containing the Tn903 kanamycin resistance gene. This new plasmid was designated pKB1429. A pBR322 derivative called pKB 701 (ATCC39772) (see U.S. Ser. No. 06/757,014, hereby incorporated by reference) was generated in which the MboI and TThIII 1 flanking the origin of replication were converted to KpnI sites. The SalI to EcoRI fragment containing serA::KanR from pKB1429 was cloned into pKB701, giving rise to pKB1438. pKB1438 was digested with KpnI to remove the ori region. The large fragment containing the ampicillin resistance coding region, as well as the serA::KanR, was circularized and used in a CaCl₂ transformation of YMC9. Following transformation, the host YMC9 cells were placed under selection on ampicillin. Under these conditions, ampicillin resistant clones develop by incorporation of the circular DNA through homologous recombination in the serA gene flanking regions. Growth of the ampicillin resistant isolate in the absence of ampicillin selection results in loss of the ampicillin resistance gene by homologous recombination of the repeated sequences of serA gene flanking regions. Such strains were identified by the loss of production of β-lactamase using AmpScreen (BRL) according to the manufacturer's directions. Duplicate streaking of single colonies on media in the presence and absence of serine revealed ampicillin sensitive clones requiring serine for growth on minimal medium and which were also resistant to kanamycin. One such isolate was named KB875.

EXAMPLE 3

Chromosomal Integration of Altered serA Sequences by Allele Exchange

The serA 1455 allele was introduced to the chromosome by a process analogous to that used for the introduction of serA::KanR as described in Example 2. Briefly, a fragment (SalI to HindIII) bearing the serA 1455 allele was cloned into pKB701. The plasmid origin was removed by KpnI digestion. The circularized DNA was used to transform to ampicillin resistance giving rise to a strain designated.

After non-selective growth, using Ampscreen and replica plating for kanamycin, KB904 (serA 1455) was isolated and shown to be sensitive to ampicillin and kanamycin KB904. The resulting serA 1455 allele can be transferred into production strains by P1 transduction. Miller, Experiments in Mol. Genetics, Cold Spring Harbor Press, pp. 201-205 (1972).

EXAMPLE 4

Chromosomal Integration of Altered serA Sequences by recD Dependent Gene Replacement.

Another approach was utilized to move the serA1508 allele on to the chromosome. The strain KB875 was made recD by P1 transduction from V220 (recD, argA:Tn10. Amundsen et al., Proc. Acad. Sci., U.S.A., 82:5558-5562 [1986]) (DSM 6823). The gene for an essential third subunit of exonuclease V. to give JGP101. The plasmid pKB1508 was linearized and used to transform JGP101 to serine prototrophy essentially as described by Shevell et al., J. Bacteriol., 170:3294-3296 (1988), to give JGP103. The serA1508 allele can then be moved to production strains by P1 transduction. Miller et al., Experiments in Mol. Genetics, Cold Spring Harbor Lab., pp. 201-205 (1972).

Harvesting of Overproduced Metabolites

For the overproduction of serine-related metabolites, cells can be prepared which produce PGD with reduced serine sensitivity, and grown in fermentors under the appropriate conditions, in most cases to stationary phase. The cells will then be harvested and lysed and the desired metabolite prepared according to standard biochemical procedures. Conditions, principles, and references for the growth of microbes, and the harvesting of specific metabolites, are provided by Crueger and Crueger, Biotechnology: A Textbook of Industrial Biology (1982) and Herrmann and Somerville, Amino Acids: Biosynthesis and Genetic Regulation (1983), incorporated herein by reference.

While several embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 18                                                  (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 1233 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: other DNA                                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        ATGGCAAAGGTATCGCTGGAGAAAGACAAGATTAAGTTTCTGCTGGTA48                             MetAlaLysValSerLeuGluLysAspLysIleLysPheLeuLeuVal                               151015                                                                         GAAGGCGTGCACCAAAAGGCGCTGGAAAGCCTTCGTGCAGCTGGTTAC96                             GluGlyValHisGlnLysAlaLeuGluSerLeuArgAlaAlaGlyTyr                               202530                                                                         ACCAACATCGAATTTCACAAAGGCGCGCTGGATGATGAACAATTAAAA144                            ThrAsnIleGluPheHisLysGlyAlaLeuAspAspGluGlnLeuLys                               354045                                                                         GAATCCATCCGCGATGCCCACTTCATCGGCCTGCGATCCCGTACCCAT192                            GluSerIleArgAspAlaHisPheIleGlyLeuArgSerArgThrHis                               505560                                                                         CTGACTGAAGACGTGATCAACGCCGCAGAAAAACTGGTCGCTATTGGC240                            LeuThrGluAspValIleAsnAlaAlaGluLysLeuValAlaIleGly                               65707580                                                                       TGTTTCTGTATCGGAACAAACCAGGTTGATCTGGATGCGGCGGCAAAG288                            CysPheCysIleGlyThrAsnGlnValAspLeuAspAlaAlaAlaLys                               859095                                                                         CGCGGGATCCCGGTATTTAACGCACCGTTCTCAAATACGCGCTCTGTT336                            ArgGlyIleProValPheAsnAlaProPheSerAsnThrArgSerVal                               100105110                                                                      GCGGAGCTGGTGATTGGCGAACTGCTGCTGCTATTGCGCGGCGTGCCG384                            AlaGluLeuValIleGlyGluLeuLeuLeuLeuLeuArgGlyValPro                               115120125                                                                      GAAGCCAATGCTAAAGCGCACCGTGGCGTGTGGAACAAACTGGCGGCG432                            GluAlaAsnAlaLysAlaHisArgGlyValTrpAsnLysLeuAlaAla                               130135140                                                                      GGTTCTTTTGAAGCGCGCGGCAAAAAGCTGGGTATCATCGGCTACGGT480                            GlySerPheGluAlaArgGlyLysLysLeuGlyIleIleGlyTyrGly                               145150155160                                                                   CATATTGGTACGCAATTGGGCATTCTGGCTGAATCGCTGGGAATGTAT528                            HisIleGlyThrGlnLeuGlyIleLeuAlaGluSerLeuGlyMetTyr                               165170175                                                                      GTTTACTTTTATGATATTGAAAATAAACTGCCGCTGGGCAACGCCACT576                            ValTyrPheTyrAspIleGluAsnLysLeuProLeuGlyAsnAlaThr                               180185190                                                                      CAGGTACAGCATCTTTCTGACCTGCTGAATATGAGCGATGTGGTGAGT624                            GlnValGlnHisLeuSerAspLeuLeuAsnMetSerAspValValSer                               195200205                                                                      CTGCATGTACCAGAGAATCCGTCCACCAAAAATATGATGGGCGCGAAA672                            LeuHisValProGluAsnProSerThrLysAsnMetMetGlyAlaLys                               210215220                                                                      GAAATTTCACTAATGAAGCCCGGCTCGCTGCTGATTAATGCTTCGCGC720                            GluIleSerLeuMetLysProGlySerLeuLeuIleAsnAlaSerArg                               225230235240                                                                   GGTACTGTGGTGGATATTCCGGCGCTGTGTGATGCGCTGGCGAGCAAA768                            GlyThrValValAspIleProAlaLeuCysAspAlaLeuAlaSerLys                               245250255                                                                      CATCTGGCGGGGGCGGCAATCGACGTATTCCCGACGGAACCGGCGACC816                            HisLeuAlaGlyAlaAlaIleAspValPheProThrGluProAlaThr                               260265270                                                                      AATAGCGATCCATTTACCTCTCCGCTGTGTGAATTCGACAACGTCCTT864                            AsnSerAspProPheThrSerProLeuCysGluPheAspAsnValLeu                               275280285                                                                      CTGACGCCACACATTGGCGGTTCGACTCAGGAAGCGCAGGAGAATATC912                            LeuThrProHisIleGlyGlySerThrGlnGluAlaGlnGluAsnIle                               290295300                                                                      GGCCTGGAAGTTGCGGGTAAATTGATCAAGTATTCTGACAATGGCTCA960                            GlyLeuGluValAlaGlyLysLeuIleLysTyrSerAspAsnGlySer                               305310315320                                                                   ACGCTCTCTGCGGTGAACTTCCCGGAAGTCTCGCTGCCACTGCACGGT1008                           ThrLeuSerAlaValAsnPheProGluValSerLeuProLeuHisGly                               325330335                                                                      GGGCGTCGTCTGATGCACATCCACGAAAACCGTCCGGGCGTGCTAACT1056                           GlyArgArgLeuMetHisIleHisGluAsnArgProGlyValLeuThr                               340345350                                                                      GCGCTGAACAAAATCTTCGCCGAGCAGGGCGTCAACATCGCCGCGCAA1104                           AlaLeuAsnLysIlePheAlaGluGlnGlyValAsnIleAlaAlaGln                               355360365                                                                      TATCTGCAAACTTCCGCCCAGATGGGTTATGTGGTTATTGATATTGAA1152                           TyrLeuGlnThrSerAlaGlnMetGlyTyrValValIleAspIleGlu                               370375380                                                                      GCCGACGAAGACGTTGCCGAAAAAGCGCTGCAGGCAATGAAAGCTATT1200                           AlaAspGluAspValAlaGluLysAlaLeuGlnAlaMetLysAlaIle                               385390395400                                                                   CCGGGTACCATTCGCGCCCGTCTGCTGTACTAA1233                                          ProGlyThrIleArgAlaArgLeuLeuTyrEnd                                              405410                                                                         (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 52 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        AlaGluGlnGlyValAsnIleAlaAlaGlnTyrLeuGlnThrSerAla                               151015                                                                         GlnMetGlyTyrValValIleAspIleGluAlaAspGluAspValAla                               202530                                                                         GluLysAlaLeuGlnAlaMetLysAlaIleProGlyThrIleArgAla                               354045                                                                         ArgLeuLeuTyr                                                                   50                                                                             (2) INFORMATION FOR SEQ ID NO:3:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 47 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                        AlaGluGlnGlyValAsnIleAlaAlaGlnTyrLeuGlnThrSerAla                               151015                                                                         GlnMetGlyTyrValValIleAspIleGluAlaAspGluAspValAla                               202530                                                                         GluLysAlaLeuGlnAlaMetLysAlaIleProAlaSerLeuAsp                                  354045                                                                         (2) INFORMATION FOR SEQ ID NO:4:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                        AlaGluGlnGlyValLeuVal                                                          15                                                                             (2) INFORMATION FOR SEQ ID NO:5:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                        AlaGluGlnGlyValLeu                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:6:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 56 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                        AlaGluGlnGlyValCysSerArgAlaAsnIleAlaAlaGlnTyrLeu                               151015                                                                         GlnThrSerAlaGlnMetGlyTyrValValIleAspIleGluAlaAsp                               202530                                                                         GluAspValAlaGluLysAlaLeuGlnAlaMetLysAlaIleProGly                               354045                                                                         ThrIleArgAlaArgLeuLeuTyr                                                       5055                                                                           (2) INFORMATION FOR SEQ ID NO:7:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 54 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                        AlaGluGlnGlyValAsnIleAlaAlaGlnTyrLeuGlnThrSerAla                               151015                                                                         GlnMetGlyTyrValValIleAspIleGluAlaAspGluAspValAla                               202530                                                                         GluLysAlaLeuSerArgGlnAlaMetLysAlaIleProGlyThrIle                               354045                                                                         ArgAlaArgLeuLeuTyr                                                             50                                                                             (2) INFORMATION FOR SEQ ID NO:8:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 37 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                        AlaGluGlnGlyValAsnIleAlaAlaGlnTyrLeuGlnThrSerAla                               151015                                                                         GlnMetGlyTyrValValIleAspIleGluAlaAspGluAspValAla                               202530                                                                         GluLysAlaLeuLeu                                                                35                                                                             (2) INFORMATION FOR SEQ ID NO:9:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 53 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                        AlaGluGlnGlyValAsnIleAlaAlaGlnTyrLeuGlnThrSerAla                               151015                                                                         GlnMetGlyTyrValValIleAspIleGluAlaAspGluAspValAla                               202530                                                                         GluLysAlaLeuGlnAlaMetLysAlaIleProGlyThrAlaIleArg                               354045                                                                         AlaArgLeuLeuTyr                                                                50                                                                             (2) INFORMATION FOR SEQ ID NO:10:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 45 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:                                       AlaGluGlnGlyValAsnIleAlaAlaGlnTyrLeuGlnThrSerAla                               151015                                                                         GlnMetGlyTyrValValIleAspIleGluAlaAspGluAspValAla                               202530                                                                         GluLysAlaLeuGlnAlaMetLysAlaIleProValLeu                                        354045                                                                         (2) INFORMATION FOR SEQ ID NO:11:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 16 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:                                       AlaGluGlnGlyValCysSerArgAlaIleArgAlaArgLeuLeuTyr                               151015                                                                         (2) INFORMATION FOR SEQ ID NO:12:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 46 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:                                       AlaGluGlnGlyValAsnIleAlaAlaGlnTyrLeuGlnThrSerAla                               151015                                                                         GlnMetGlyTyrValValIleAspIleGluAlaAspGluAspValAla                               202530                                                                         GluLysAlaLeuSerArgAlaIleArgAlaArgLeuLeuTyr                                     354045                                                                         (2) INFORMATION FOR SEQ ID NO:13:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 14 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:                                       CTAGTCTAGACTAG14                                                               (2) INFORMATION FOR SEQ ID NO:14:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 8 base pairs                                                       (B) TYPE: neucleic acid                                                        (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:                                       CTCTAGAG8                                                                      (2) INFORMATION FOR SEQ ID NO:15:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 12 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:                                       TGCTCTAGAGCA12                                                                 (2) INFORMATION FOR SEQ ID NO:16:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 10 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:16:                                       GCTCTAGAGC10                                                                   (2) INFORMATION FOR SEQ ID NO:17:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 11 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:17:                                       GCTCTAGAGCA11                                                                  (2) INFORMATION FOR SEQ ID NO:18:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 11 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:18:                                       TGCTCTAGAGC11                                                                  __________________________________________________________________________ 

I claim:
 1. Engineered DNA encoding 3-phosphoglycerate dehydrogenase (PGD) from E. Coli with reduced sensitivity to inhibition by serine in comparison to wild-type PGD;said DNA encoding PGD from E. Coli comprising a C-terminal insertion into a wild-type PGD sequence; wherein said insertion is between VAL 363 and ASN 364 of wild-type PGD of (SEQ ID NO:1) or between ALA 392 and GLN 394 of wild-type PGD (SEQ ID NO:1).
 2. An expression vector comprisingthe engineered DNA of claim 1 and regulatory DNA positioned and oriented for expressing said engineered DNA in a host expression system.
 3. A cell comprising the engineered DNA of claim 1, and regulatory DNA positioned and oriented to express said engineered DNA in said cell.
 4. The cell of claim 3, in which said cell is deleted for wild-type serA.
 5. A method for producing a product selected from the group consisting of serine and a serine-derived product, comprisingculturing a cell according to claim 3; and recovering said product.
 6. The method of claim 5, in which said product is serine.
 7. A 3-phosphoglycerate dehydrogenase having the amino acid sequence encoded by the engineered DNA of claim
 1. 8. Engineered DNA encoding 3-phosphoglycerate dehydrogenase (PGD) from E. coli with reduced sensitivity to inhibition by serine in comparison to wild-type PGD;said DNA encodes PGD from E. coli having an amino acid sequence selected from the group consisting of the following (SEQ ID NO.: 3); (SEQ ID NO.: 4); (SEQ ID NO.: 5); (SEQ ID NO.: 6); (SEQ ID NO.: 7); (SEQ ID NO.: 8); (SEQ ID NO.: 9); (SEQ ID NO.: 10); (SEQ ID NO.: 11); and (SEQ ID NO.: 12) instead of deleted 52 C-terminal amino acids of the wild-type PGD of (SEQ ID NO.: 1).
 9. A 3-phosphoglycerate dehydrogenase having the amino acid sequence encoded by the engineered DNA of claim
 8. 